No abstract

It is an axiom in ecology that knowing the sheer number of individuals in a population is of very little help if the objective is to understand future and past changes in population size. Yet, this is exactly how migratory European ducks are monitored, many of which are important quarry species in several countries. We argue that present monitoring is insufficient to address objectives of wise use and sustainability such as those emphasised in recent management directives and multilateral international agreements. The two main problems are the almost total lack of reliable data on recruitment and mortality. We advocate a pan-European monitoring system based on undisputed scientific principles; i.e. a long-term, coordinated and standardised scheme that produces data about vital rates of duck populations as well as about harvest size. Data from such a scheme can be used by game biologists to produce predictive tools, thus providing a functional basis for management decisions for adaptive harvesting and conservation alike.

Captive-reared animals used in reinforcement programs are generally less likely to survive than wild conspecifics. Digestion efficiency and naive behaviour are two likely reasons for this pattern. The Mallard is a species with high adaptability to its environment and in which massive reinforcement programs are carried out. We studied physiological and behavioural factors potentially affecting body condition and survival of captive-reared Mallards after being released. Digestive system morphology and an index of body condition were compared among three groups: captive-reared birds remaining in a farm (control), captive-reared birds released into the wild as juveniles (released) and wild-born birds (wild). We also compared behaviour and diet of released vs. wild Mallards. Finally, we conducted a 1-year survival analysis of captive-reared birds after release in a hunting-free area. Gizzard weight was lower in control Mallards, but the size of other organs did not differ between controls and wild birds. The difference in gizzard weight between released and wild birds disappeared after some time in the wild. Diet analyses suggest that released Mallards show a greater preference than wild for anthropogenic food (waste grain, bait). Despite similar time-budgets, released Mallards never attained the body condition of wild birds. As a consequence, survival probability in released Mallards was low, especially when food provisioning was stopped and during harsh winter periods. We argue that the low survival of released Mallards likely has a physiological rather than a behavioural (foraging) origin. In any case, extremely few released birds live long enough to potentially enter the breeding population, even without hunting. In the context of massive releases presently carried out for hunting purposes, our study indicates a low likelihood for genetic introgression by captive-reared birds into the wild population.

Understanding and modeling population change is urgently needed to predict effects of climate change on biodiversity. High trophic-level organisms are influenced by fluctuations of prey quality and abundance, which themselves may depend on climate oscillations. Modeling effects of such fluctuations is challenging because prey populations may vary with multiple climate oscillations occurring at different time scales. The analysis of a 28-yr time series of capture-recapture data of a tropical seabird, the Nazca Booby (Sula granti), in the Galápagos, Ecuador, allowed us to test for demographic effects of two major ocean oscillations occurring at distinct time-scales: the inter-annual El Niño Southern Oscillation (ENSO) and inter-decadal oscillations. As expected for a tropical seabird, survival of fledgling birds was highly affected by extreme ENSO events; by contrast, neither recruitment nor breeding participation were affected by either ENSO or decadal oscillations. More interesting, adult survival, a demographic trait that canalizes response to environmental variations, was unaffected by inter-annual ENSO oscillations yet was shaped by the Pacific Decadal Oscillation and small pelagic fish regime. Adult survival decreased during oceanic conditions associated with higher breeding success, an association probably mediated in this species by costs of reproduction that reduce survival when breeding attempts end later. To our knowledge, this is the first study suggesting that survival of a vertebrate can be vulnerable to a natural multidecadal oscillation.

Disruption of naturally evolved spatial patterns of genetic variation and local adaptations is a growing concern in wildlife management and conservation. During the last decade, releases of native taxa with potentially non-native genotypes have received increased attention. This has mostly concerned conservation programs, but releases are also widely carried out to boost harvest opportunities. The mallard, Anas platyrhynchos, is one of few terrestrial migratory vertebrates subjected to large-scale releases for hunting purposes. It is the most numerous and widespread duck in the world, yet each year more than three million farmed mallard ducklings are released into the wild in the European Union alone to increase the harvestable population. This study aimed to determine the genetic effects of such large-scale releases of a native species, specifically if wild and released farmed mallards differ genetically among subpopulations in Europe, if there are signs of admixture between the two groups, if the genetic structure of the wild mallard population has changed since large-scale releases began in the 1970s, and if the current data matches global patterns across the Northern hemisphere. We used Bayesian clustering (STRUCTURE software) and Discriminant Analysis of Principal Components (DAPC) to analyze the genetic structure of historical and present-day wild (n = 171 and n = 209, respectively) as well as farmed (n = 211) mallards from six European countries as inferred by 360 single-nucleotide polymorphisms (SNPs). Both methods showed a clear genetic differentiation between wild and farmed mallards. Admixed individuals were found in the present-day wild population, implicating introgression of farmed genotypes into wild mallards despite low survival among released farmed mallards. Such cryptic introgression would alter the genetic composition of wild populations and may have unknown long-term consequences for conservation.

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Summary 1.We sometimes need to predict the maximum number of bird-days that can be supported by the food supply in a site used by migratory birds outside the breeding season. So defined, carrying capacity is often estimated using the daily ration model (DRM). In this, the total biomass of accessible food, aggregated across all patches of differing food density, is divided by an individual's daily requirement. Carrying capacity can also be estimated using spatial depletion models (SDM), in which patches of differing food density are treated separately. We identify here some of the features of the food supply that enable patches to be amalgamated so that the very simple DRM can be used instead of a more complex SDM. 2. We show by theoretical modelling that the predictions of the DRM and SDM are often the same even though initial food density varies between patches. The wide range of conditions over which this is so are specified. 3. A DRM and a SDM of wildfowl eating seagrass Zostera spp. in a nature reserve produced similar predictions for the number of bird-days supported intertidally before the birds switched to farmland. 4. We conclude that a DRM can often be used instead of a SDM to predict bird-day carrying capacity. We identified two conditions in which the DRM cannot be used: (i) when the rate of additional food loss due to factors other than depletion by the birds themselves differs between patches; and (ii) when the relative profitabilities of patches, and thus the number of birds using each patch, change through the depletion period in non-simple ways that cannot be predicted without a SDM. Examples of such exceptions are described. Synthesis and applications.By showing when a DRM can be used instead of a SDM, this study should help nature managers to predict the bird-day carrying capacity of a site in the simplest way yet available. However, predicting the maximum number of bird-days supported is not equivalent to predicting demographic rates and should not be confused with predictions for population size. Rather, it is probably best regarded as a measure of site quality.

Captive‐bred mallards Anas platyrhynchos have been released for hunting purposes at a very large scale in Europe since the mid‐1970s. In spite of a potential genetic impact, the actual contribution of restocked mallards to the genome of the target population has received little attention. The genetic structure of modern wild mallards in the Camargue, Southern France, was assessed from two samples: one originating from shot birds in hunting bags and one from presumed wild ducks captured alive in a hunting‐free reserve. Reference samples originated from five mallard farms, as well as from museum samples collected before the mid‐1970s (i.e. before massive mallard releases started). Our results revealed that the genetic signature of wild wintering mallards has not changed significantly because museum and presumed wild samples from the Camargue hunting‐free nature reserve were genetically similar, and clearly differentiated from the farm mallards. This suggests that mallard releases in the Camargue or elsewhere in France, although massive, have not actually translated into complete admixture of wild and captive genomes, most likely due to low survival of released birds once in the wild. Nevertheless, although genetic introgression of the wild population by captive‐bred was contained, we found significant rates of hybridization between wild and captive‐bred mallards in modern samples. This result suggests that long‐term releases of captive‐bred mallards, if carried on at such large scale, could compromise irreversibly the genetic structure and composition of European mallards. This work contributes to fill in the gap on the monitoring of the genetic consequences of large‐scale game releases for exploitation.